The vagus nerve is one of the twelve cranial nerves that originate in the brain. The nerve fibers in the vagus carry information to and from the brain to the larynx, the heart, the stomach, lungs and esophagus, and may also be connected to the limbic structures in the brain, which are associated with the regulation of mood. The vagus nerve, therefore, plays a key role in neural control of body functions, e.g. neural, cardiovascular, respiratory and metabolic functions, and therapies which include stimulation of the vagus nerve have been developed to provide therapy for a number of diseases including epilepsy, anxiety disorders, obesity, pain, cognition and memory disorders as well as cardiovascular, sleep, and breathing disorders.
It has been shown, for example, that the introduction of stimulation control signals to the vagus nerve can modify hyperactivity in the brain, and therefore provide therapeutic benefits for the diseases and conditions described above. The superposition of such corrective measures inhibits the normal progress of a seizure and can prevent seizures altogether. It is also possible that control signals of proper magnitudes when applied to associated neural tracts can cause neural activity to return to a normal state. Corrective signals of this type can be generated by appropriate electrical pulses or waves applied to neurons.
To provide the appropriate stimulus of the vagus nerve, implantable vagus nerve stimulators (VNS), such as those described in U.S. Pat. No. 4,702,254, have been developed. These devices include a pulse generator which is implanted subcutaneously in the patient, and is connected to the vagus nerve with an electrode or electrodes, which are also implanted in the patient. One such system, for example, is the commercially-available VNS therapy system produced by Cyberonics® of Houston, Tex., which consists of a pulse generator, electrode leads, and a programming wand for programming the pulses generated. The pulse generator is surgically implanted in subcutaneous tissue in the upper chest region. The lead is placed in a subcutaneous tunnel that runs from the pulse generator to the vagus nerve, and an electrode connects the lead to the vagus nerve. The generator can be telemetrically interrogated and programmed using a programming wand and associated software as described, for example, in U.S. Pat. No. 5,222,494 to Baker, and U.S. Pat. No. 5,928,272 to Adkins. The programming wand, therefore, can be used to establish a therapeutic stimulation for the patient. It can also be used, in a special mode, to provide a signal indicating the onset of stimulation. The special mode, however, requires operation of a frequency of 5 Hz or below, whereas stimulation frequencies are typically 20-30 Hz. Furthermore, the data is provided in an asynchronous serial link and cannot, therefore, be monitored in conjunction with other physiological signals.
While these commercially-available systems have therefore been successful in providing therapeutic pulses to the vagus nerve to provide therapy for mitigating the effects of a number of diseases, these devices present certain difficulties as they do not provide any means for monitoring the application of a therapeutic stimulating pulse once the device is implanted. Because it is not possible to monitor the pulse as it is applied, it is difficult for care providers to correlate the stimulation of the vagus nerve with the response of the patient, which can be manifested as a physiological change in the patient (i.e. ECG, EEG, blood pressure, breathing, etc.). Because of these problems, it is difficult for care providers to establish an appropriate dose-response relationship for VNS therapy, and to monitor the success of a treatment. Adjustment of the VNS device settings for an individual patient is therefore a major clinical issue.
Moreover, while devices for non-invasive monitoring of biological signals are available, these devices are typically intended for use in monitoring signals such as, for example, electrocardiogram (ECG) and/or electroencephalogram (EEG) signals. The EEG or ECG signals, however, vary significantly from those produced by a VNS stimulator device, and these devices therefore cannot be used to monitor VNS pulses. EEG and ECG signals, for example, are continuous biological signals as opposed to artificial VNS pulses, have a significantly higher amplitude than VNS signals, and are much lower in frequency than VNS signals. Techniques commonly employed for EEG and ECG monitors therefore are not useful in monitoring VNS signals.